Method of forming a sample image and charged particle beam apparatus

ABSTRACT

An object of the present invention is to provide a sample image forming method and a charged particle beam apparatus which are suitable for realizing suppressing of the view area displacement with high accuracy while the influence of charging due to irradiation of the charged particle beam is being suppressed.  
     In order to attain the above object, the present invention provide a method of forming a sample image by scanning a charged particle beam on a sample and forming an image based on secondary signals emitted from the sample, the method comprising the steps of forming a plurality of composite images by superposing a plurality of images obtained by a plurality of scanning times; and forming a further composite image by correcting positional displacements among the plurality of composite images and superposing the plurality of composite images, and a charged particle beam apparatus for realizing the above method.

TECHNICAL FIELD

[0001] The present invention relates to a method of forming a sampleimage and a charged particle beam apparatus, and particularly to amethod of forming a sample image and a charged particle beam apparatuswhich are suitable for obtaining a high resolution image in a highmagnification and not influenced by image drift.

BACGROUND ART

[0002] In a charged particle beam apparatus typical of which is ascanning electron microscope, desired information (for example, a sampleimage) is obtained from a sample by scanning a thinly converged chargedparticle beam on the sample. In such a charged particle beam apparatus,the resolution becomes higher year by year, and the required observationmagnification becomes higher as the resolution becomes higher. As thebeam scanning method for obtaining a sample image, there are a methodwhich obtains a final objective image by adding a plurality of imagesobtained by high speed scanning and a method which obtains a finalobjective image by once of low speed scanning (acquiring time of oneframe image: approximately 40 seconds to 80 seconds). The influence ofthe drift of a view area on the acquired image becomes more serious asthe observation magnification becomes higher. For example, in the methodof acquiring the objective image by adding image signals obtained by thehigh speed scanning pixel by pixel (frame addition), when there is driftcaused by charge-up of the sample during adding the images, theobjective image after adding has blurs in a direction of the driftbecause displaced pixels of the view area are added. Reducing theinfluence of the drift may be attained by reducing number of addingframes and shortening the adding time, but this method can not obtain asufficient S/N ratio.

[0003] On the other hand, in the method of acquiring the image by thelow speed scanning, when there is drift during acquiring the image, theimage is deformed because the view area flows in a direction of thedrift.

[0004] A technology is disclosed in Japanese Patent ApplicationLaid-Open No.62-43050. The technology is that a pattern for detectingdrift is stored, and a beam irradiating position is corrected byperiodically acquiring an image of the pattern to detect a displacementbetween the acquired image and the stored pattern.

[0005] A technology is disclosed in Japanese Patent ApplicationLaid-Open No.5-290787. The technology is that two images are acquiredbased on electron beam scanning on a specified observed area, andpattern matching is performed in order to specify an amount ofdisplacement and a direction of displacement between the both images,and pixels are added by moving the pixels by the specified amount ofdisplacement and the specified direction of displacement.

[0006] In the technology disclosed in Japanese Patent ApplicationLaid-Open No.62-43050, the accuracy of controlling the beam irradiatingposition becomes insufficient when the observation magnification becomesseveral hundred thousand times. For example, when an image of 1280×960pixels is tried to be acquired with an observation magnification of 200thousand times, the size of one pixel on the observation view area (onthe sample) is approximately 0.5 nm. Measurement and evaluation with ahigher magnification become necessary as the scale-down of a measuredobject is progressed. Under such a condition, when the technology isapplied to an apparatus for forming a final image by adding a pluralityof images, image shift (drift) below several nm causes “blurs” in aflame added image.

[0007] Although the technology disclosed in Japanese Patent ApplicationLaid-Open No.62-43050 suppresses the image shift by controlling thescanning position of the electron beam to correct the drift, thecorrecting accuracy of the position by such control is limited toseveral nm to several tens nm. Accordingly, it is almost impossible tocorrect the position (correct the drift) of an image having amagnification the position above several hundred thousand times with apixel level. In addition, there is a problem in that the through-put isdecreased because stabilization of the drift takes a long time.

[0008] On the other hand, the technology disclosed in Japanese PatentApplication Laid-Open No.5-290787 can be appreciated in the point thatthe position between the images can be corrected in the pixel level, butthere is the following problem.

DISCLOSURE OF THE INVENTION

[0009] Because an S/N ratio of image data before processing image addingis low and accordingly the displacement between the images is difficultto be detected, it is difficult to correct the displacement with highaccuracy. Further, it can be considered that the S/N ratio is improvedby increasing the probe current (the electron beam current) to increasethe amount of secondary electron emission. However, in a case of aneasily charged sample, the displacement between the images acquired atdifferent timing is further increased by movement of the view area ofthe electron beam due to charging, and as the result, it has beendifficult to correct the displacement with high accuracy. Furthermore,in a case where a sample weak against electron beam damage is irradiatedby an electron beam having a large beam current, there is a problem inthat the sample may be broken or evaporated.

[0010] An object of the present invention is to provide a sample imageforming method and a charged particle beam apparatus which are suitablefor realizing suppressing of the view area displacement with highaccuracy while the influence of charging due to irradiation of thecharged particle beam is being suppressed.

[0011] In order to attain the above object, the present inventionprovide a method of forming a sample image by scanning a chargedparticle beam on a sample and forming an image based on secondarysignals emitted from the sample, the method comprising the steps offorming a plurality of composite images by superposing a plurality ofimages obtained by a plurality of scanning times; and forming a furthercomposite image by correcting positional displacements among theplurality of composite images and superposing the plurality of compositeimages, and a charged particle beam apparatus for realizing the abovemethod.

[0012] As described above, since positional displacements can bedetected among images having a sufficient S/N ratio without increasingbeam current by forming composite images and then correcting thepositional displacements, “blurs” of an image at adding the frames canbe suppressed because the positional displacements are corrected withhigh accuracy. The other objects of the present invention and the otherdetailed construction of the present invention will be described in thesection “DESCRIPTION OF THE PREFERRED EMBODIMENTS” in the presentspecification.

BRIEF DESCRIPTION OF DRAWINGS

[0013]FIG. 1 is a block diagram showing a scanning electron microscopefor explaining an embodiment in accordance with the present invention.

[0014]FIG. 2 is a flowchart showing the processing of reconstructing anobjective image by correcting positional displacements of a plurality ofacquired images.

[0015]FIG. 3 is photographs showing an image obtained by simply addingimages and an image obtained by correcting positional displacementsafter acquiring a plurality of images and then adding the positionaldisplacement corrected images.

[0016]FIG. 4 is a flowchart showing the processing combining of theprocessing of correcting the drift by controlling the beam irradiatingposition or the sample position and the processing of correcting thepositional displacements of a plurality of acquired images and thenadding the plurality of images of which the positional displacements arecorrected.

[0017]FIG. 5 is a conceptual view showing the process of adding aplurality of images while positional displacements among the pluralityof images are being corrected.

[0018]FIG. 6 is a flowchart showing the processing of restoringdeformation of an image acquired by slow scanning, the deformation beingcaused by the drift.

[0019]FIG. 7 is a conceptual view showing the process of restoring thedeformation of the image acquired by slow scanning, the deformationbeing caused by the drift.

[0020]FIG. 8 is a conceptual view showing the process of correcting thepositional displacements among a plurality of profiles obtained throughline scanning and then adding the plurality of images of which thepositional displacements are corrected.

[0021]FIG. 9 is a graph showing an example of estimating the beam damagefrom measured length values in a plurality of acquired images tocalculate a measured value of length which is not influenced by beamdamage.

[0022]FIG. 10 is a view showing an example in which a plurality ofimages each having a region wider than a view area of an objective imageare acquired, and after adding the images of which the view areadisplacements among the images are corrected, the region of theobjective view area in the central portion is cut out.

[0023]FIG. 11 is a view showing an example in which an image having adetected abnormality is removed out of a plurality of acquired images,and then the images excluded the abnormal image are added aftercorrecting the view area displacement.

[0024]FIG. 12 is a view showing an example in which view areadisplacements among a plurality of images acquired by detecting aplurality of image signals at a time are corrected, and then the imagesare added.

[0025]FIG. 13 is a view showing an example in which positionaldisplacements of a plurality of images are corrected only in a specifieddirection, and then the images are added.

[0026]FIG. 14 is a view explaining a method of detecting the amount ofpositional displacement and adding the corrected images.

[0027]FIG. 15 is a view explaining another method of detecting theamount of positional displacement and adding the corrected images.

[0028]FIG. 16 is a view showing an example of a GUI page displayed on animage displaying unit.

[0029]FIG. 17 is a view showing an example of an electron detectingsystem in an embodiment of a charged particle beam apparatus inaccordance with the present invention.

[0030]FIG. 18 is a view showing another example of an electron detectingsystem in an embodiment of a charged particle beam apparatus inaccordance with the present invention.

[0031]FIG. 19 is a view showing an example of a GUI page displayed on animage displaying unit.

[0032]FIG. 20 is views for explaining the principle of Embodiment 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] Embodiments of the present invention will be described below,referring to the accompanied drawings.

[0034]FIG. 1 is a block diagram showing an embodiment of a scanningelectron microscope in accordance with the present invention. A voltageis applied between a cathode 1 and a first anode 2 by a high voltagecontrol power source 20 controlled by a computer 40 to extract a primaryelectron beam 4 with a preset emission current from the cathode 1. Anacceleration voltage is applied between the cathode 1 and a second anode3 by the high voltage control power source 20 controlled by the computer40, and the primary electron beam 1 emitted from the cathode 1 isaccelerated and travels to a lens system in the rear stage.

[0035] The primary electron beam 4 is focused by a focusing lens 5controlled by a lens control power source 21. Then, after unnecessaryregions of the primary electron beam are removed by an aperture plate 8,the primary electron beam 4 is focused on a sample 10 as a very smallspot by a focusing lens 6 controlled by a lens control power source 22and an objective lens 7 controlled by an objective lens control powersource 23. The objective lens 7 may be of various type such as anin-lens type, an out-lens type, a snorkel type (a semi-in-lens type)etc. Further, each of the lenses may be constructed of an electrostaticlens which is composed of a plurality of electrodes.

[0036] The primary electron beam 4 is two-dimensionally (in X-Ydirections) scanned on the sample 10 by a scanning coil 9. Current issupplied to the scanning coil 9 from a scanning coil control powersource. Secondary signals 12 generated from the sample 10 by irradiationof the primary electron beam are travel to the upper portion of theobjective lens 7, and then are separated from the primary electrons by asecondary signal separation orthogonally-crossing electro-magnetic fieldgenerator 11 to be detected by a secondary signal detector 13. Thesignals detected by the secondary signal detector 13 are amplified by asignal amplifier 14, and then transmitted to an image memory 25 anddisplayed on an image display unit 26 as a sample image. The secondarysignal detector may be a detector for detecting secondary electrons orreflected electrons, or a detector for detecting light or X-rays.

[0037] An address signal corresponding to a memory area of the imagememory 25 is generated in a computer 40, and converted to an analoguesignal, and than supplied to the scanning coil 9 though the scanningcoil control power source 24. The address signal in X-direction is adigital signal repeating, for example, 0 to 512 in a case where theimage memory 25 is 512×512 pixels, and the address signal in Y-directionis a digital signal repeating 0 to 512 which is added by 1 when theaddress signal in X-direction reaches 512 from 0. The signals areconverted to the analogue signals.

[0038] Since the address of the image memory 25 corresponds to theaddress of the reflection signal for scanning the primary electron beam,a two-dimensional image of the deflection region of the primary electronbeam by the scanning coil 9 is recorded in the image memory 25. Thesignals in the image memory 25 can be sequentially and successively readout using a read-out address generating circuit (not shown) synchronizedby a read-out clock. The signal read-out corresponding to the address isconverted to an analogue signal, and becomes a brightness modulatedsignal for the image display unit 26.

[0039] The image memory 25 has a function for superposing (adding) theimages (image data items) in order to improve the S/N ratio and thenstoring the composite image. For example, by superposing images obtainedby 8 times of two-dimensional scanning and then storing the compositeimage, one frame of complete image is formed. That is, a final image isformed by adding images which are formed by once or more times of X-Yscanning. Number of images (number of adding frames) for forming oneframe of the complete image may be arbitrarily set, and an appropriatenumber is set in taking into consideration conditions such as secondaryelectron generating efficiency and so on. Further, by superposing aplurality of frames each of which is formed by adding the plurality ofimages, a finally desired image may be formed. By executing blanking ofthe primary electron beam at the time when a desired number of imageframes are stored or after the time, information input to the imagememory may be interrupted.

[0040] Further, in a case where the number of adding frames is set to 8,it is possible to provide such a sequence that the first frame of imagemay be deleted when a ninth frame of image is input so that 8 frames ofimage remain as the result. Otherwise, it is possible to performweighted addition averaging. That is, when a ninth frame of image isinput, an added image stored in the image memory is multiplied by ⅞ andthen the ninth frame of image is added to the added image after beingmultiplied by ⅞.

[0041] A two-stage deflecting coil 51 (an image shift deflector) isarranged at a position the same as that of the scanning coil 9, andthereby, the position of the primary electron beam 4 (the observed area)on the sample 10 can be two-dimensionally controlled. The deflectingcoil 51 is controlled by a deflecting coil control power source 31.

[0042] A stage 15 can move the sample 10 at least in 2 directions(X-direction and Y-direction) on a plane normal to the primary electronbeam.

[0043] From an input unit 42, an image acquiring condition (scanningspeed, number of adding flames of image) and a method of correcting viewarea can be specified, and outputting and storing of the images can bealso specified.

[0044] Further, the embodiment of the apparatus in accordance with thepresent invention comprises a function for forming a line profile basedon detected secondary electrons or detected reflected electrons. Theline profile is formed based on an amount of detected electrons when theprimary electron beam is one-dimensionally or two-dimensionally scannedor based on brightness information of the sample image, and the obtainedline profile is used for dimension measurement of a pattern formed, forexample, on a semiconductor wafer. The embodiment of the apparatus inaccordance with the present invention may further comprise an interface41 for transmitting image data to an external unit or the like, and arecording unit 27 for storing image data to an appropriate memorymedium.

[0045] In the explanation of FIG. 1, the control unit is described as aunit integrated with the scanning electron microscope or the like, butit is, of course, not limited to such a unit. A control processorseparately provided from the scanning electron microscope may be used toexecute the processing as described below. At that time, a transmittingmedium for transmitting signals from the control processor to thescanning electron microscope and input and output terminals forinputting and outputting the transmitted signals through thetransmitting medium are necessary.

[0046] Further, it is possible that a program for executing theprocessing to be described below is registered in a memory medium, andthe program is executed by the control processor for supplying necessarysignals to the scanning electron microscope having an image memory. Thatis, the embodiments of the present invention to be described below alsohold as the invention of program which can be employed to a chargedparticle beam apparatus such as a scanning electron microscope having animage processor.

[0047] (Embodiment 1)

[0048] In an embodiment of a method of improving an S/N ratio by addingTV scanned images, the processing flow of FIG. 2 will be described belowin detail. FIG. 5 is a view schematically showing the processing of FIG.2.

[0049] First Step (S2001):

[0050] Number N0 of adding frames for each acquired image and number N1of acquired image sheets are specified. At that time, total number ofadding frames of the final image is N0×N1. In general, by setting thenumber N0 to 2 frames to 8 frames and the number N1 to 10 sheets to 50sheets, a necessary S/N ratio can be obtained depending on the purpose.In a case where each of image is acquired with slow scanning slightlyslower than the TV scanning, the number NO may be set to 1 frame. In acase of TV scanning of interlace type, the number NO can be set to 2. Inregard to the condition setting, it is preferable that the plurality ofsample images are formed by fixing the optical conditions (a focusingcondition of the electron beam and a scanning condition) in order tomake detection of positional displacement easy.

[0051] Second Step (S2002):

[0052] As starting of acquiring image is instructed from the input unit42, N1 sheets of images of frame adding number NO (F1, F2, . . . , FN1)in the same view area are successively acquired.

[0053] Third Step (S2003):

[0054] F1 is set to a memory area of the objective image F0.

[0055] Forth Step (S2004):

[0056] A sharpened image F0 a is produced from the objective image F0.As the sharpening processing, a technique using an image filter foremphasizing edges in the image may be used.

[0057] Fifth Step (S2005):

[0058] A sharpened image F2 a is produced from the image F2.

[0059] Sixth Step (S2006):

[0060] A positional displacement between the sharpened image F2 a of F2and the sharpened image F0 a is detected. Calculation processing such asimage correlation may be applied to the detection of the positionaldisplacement. However, of course, the present invention is not limitedto the above, and all the image processing methods capable of detectingthe positional displacement are applicable.

[0061] Seventh Step (S2007):

[0062] Pixels of the original image F2 is shifted by the amount of thedisplacement of view area detected in the Sixth Step and added to theimage of F0, and then the formed image is returned as the objectiveimage F0 again.

[0063] Eighth Step (S2008):

[0064] By repeating the Fourth Step to the Sixth Step Substituting F3for F2, the adding processing with the correction of positionaldisplacement is executed to all the Ni sheets of images.

[0065] In the present embodiment, the finally obtained image is an imageformed by adding N0×N1 frames, but the image is blurred by the driftonly when N0 frames are added. Therefore, the blur of the image by thedrift is reduced to 1/N1 compared with the case of directly adding N0×N1frames. By employing such a sequence, it is possible to remove thepositional displacement in a direction on the two-dimensional imageplane between images acquired at different timing due to charge-up onthe sample, and accordingly, image blurs of the image can be suppressedor eliminated.

[0066]FIG. 3 shows an example of a result obtained by this embodiment.FIG. 3(a) is an image obtained through commonly adding the frames(1280×960 pixels, 200 thousands times of magnification), and drifts areaccumulated during adding the images to form conspicuous “blur” in thefinal image. FIG. 3(b) is an image obtained by acquiring 10 sheets ofimages having frame adding number {fraction (1/10)} times as small asthe frame adding number of FIG. 3(a), and adding these 10 sheets of theimages while the positional displacements are being corrected. In FIG.3(b), though the total frame adding number of images is the same as thatof FIG. 3(a), the “blur” in the final image caused by the drift is alsoreduced to {fraction (1/10)} times as small as that in FIG. 3(a) becauseonly the drift accumulated each of the added image becomes “blur” in thefinal image and the acquiring time for each image is {fraction (1/10)}times as short as that in FIG. 3(a).

[0067] Since the amount of drift changes depending on the kind of thesample, the optical condition and so on, it is preferable that N0 and N1are set corresponding to the S/N ratio. Since number of scanning times(number of images) required for securing a required S/N ratio isdetermined based on the quality of obtained image and the efficiency ofgenerating secondary electrons, N0 and N1 may be determined in takingthe degree of drift into consideration. Further, it is also possible toconstruct the sequence that by inputting a parameter expressingconditions of the sample (easiness of charge-up etc) and at least one oftotal adding number, number of adding frames (N0) and number of acquiredimages (N1), the other two parameters are determined. According to sucha construction, the apparatus condition can be easily set only byinputting specification necessary for observation.

[0068] In the present embodiment, although the positional displacementbetween the frame-added images is corrected, the present invention isnot limited to the above. Correction of the positional displacement maybe executed by the unit of an arbitrary number of frames or by the unitof arbitrary number of acquired sheets. At that time, unless an image tobe compared with for detecting the positional displacement has an S/Nratio larger than a certain value, the drift detecting accuracy will bedecreased. Therefore, it is preferable that number of images necessaryfor securing a desired S/N ratio is set as the frame adding number (N0),and then number of acquired image sheets (N1) for obtaining a necessaryS/N ratio for the final sample image is set.

[0069] In the present invention, the image may be stored in the imagememory 25 after correcting the positional displacement. Otherwise, bypreparing a frame memory corresponding to (frame addingnumber)×(acquired images), the positional displacement among sampleimages may be corrected when the sample image is displayed, or when thesample image is transferred to an external image memory element, orbefore the sample image is transferred to the external image memoryelement. Otherwise, the positional displacement among sample images maybe corrected in the external image memory element.

[0070] By preparing at least an image memory for storing a compositeimage, an image memory for storing images before executing superposingprocessing and an image memory for storing an image to be acquired,images acquired one after another by the electron beam scanning can besuccessively superposed.

[0071] In the present embodiment, in order to make the setting of N0 andN1 for specified samples easier, the system may be constructed in suchthat a reference image for each combination of N0 and N1 is stored, andthe reference image can be read out at setting N0 and N1. By doing so,an operator can set appropriate N0 and N1 by referring to the referenceimage.

[0072] It is preferable that when drift is fast, number of displacementcorrections is increased by decreasing number of frames N0, and thatwhen drift is nor so fast, number of frames N0 is increased in order toimprove the quality of the image to be compared with. For example, it ispreferable that as a means for appropriately setting numbers of N0 andN1, a means for adjusting N0 and N1 stepwise is provided. In a casewhere the total adding frame is set to 50, the combinations of N0 and N1are 1×50, 2×25, 5×10, 10×5, 25×2 and 50×1. However, by providing a meansfor adjusting the combination and a means for displaying an actuallyadded image, the operator can set appropriate N0 and N1 from thesuperposed image without detailed knowledge on the technology in regardto the present invention.

[0073] By providing the adjusting means described above, not only in thecase of correcting the displacement, but also in a case where thequality of image is changed by changing the combination of N0 and N1, anappropriate combination of N0 and N1 can be easily selected.

[0074] Further, the same effect can be attained by providing a means foradjusting the degree of displacement correction which sets Ni to alarger value when “the degree of displacement correction is large” isselected, and sets NO to a smaller value when “the degree ofdisplacement correction is small” is selected.

[0075] (Embodiment 2)

[0076] A processing flow of FIG. 4 will be described below in detail.

[0077] First Step (S4001):

[0078] Number N0 of adding frames for each acquired image and number N1of acquired image sheets are set.

[0079] Second Step (S4002):

[0080] Two sheets of images of the frame adding number N0 aresuccessively acquired.

[0081] Third Step (S4003):

[0082] Sharpened images are generated from the acquired two sheets ofimages, and a positional displacement between the sharpened images iscalculated.

[0083] Therein, when the amount of this displacement exceeds a presetallowable value, each of the images before correcting the position andbeing added conspicuously includes “blurs” due to drift. Therefore, theprocessing is stopped, and a display function may notify the operatorthat the drift is too large.

[0084] Fourth Step (S4004):

[0085] The two sheet of images after correcting the positionaldisplacement are added to each other, and the added image is registeredas F0.

[0086] Fifth Step (S4005):

[0087] The view area is moved in a direction canceling the positionaldisplacement obtained in the process of S4003. Therein, as the shiftingmeans, each of a method of using an electric view-area shifting means(an image shift deflector) and a method of using a stage is availabledepending on the amount of shifting. In general, when the amount ofshifting is small, both of the image shift deflector and the stage areused. When the amount of shifting is large, the stage is used or theimage shift deflector is used if necessary. By canceling thedisplacement of the view area using image shift deflector and the stage,the displacement between the images can be compressed even if there is acomparatively large drift. Therefore, it is possible to solve theproblem that an effective view area (an area where view areas of imagesare overlapped with one another) after correcting the positionaldisplacement by the image processing becomes narrow.

[0088] Sixth Step (S4006):

[0089] The next image is acquired.

[0090] Seventh Step (S4007):

[0091] By forming a sharpened image of the acquired image and asharpened image of F0, a positional displacement between the sharpenedimages is calculated.

[0092] Eighth Step (S4008):

[0093] The image F0 and the image acquired in S4006 are added bycorrecting the positional displacement between the images, and the addedimage is newly set as F0.

[0094] Ninth Step (S4009):

[0095] The view area is moved in a direction canceling the positionaldisplacement obtained in the process of S4008.

[0096] Tenth Step (S4010):

[0097] By repeating the process S4006 to the process S4009, N1 sheets ofimages are obtained, and the obtained images are added.

[0098] According to the above construction, a large drift component canbe corrected by the stage and the beam deflection, and very small driftof pixel level can be corrected at adding the images. Therefore, a highresolution image can be obtained by effectively correcting even acomparatively large drift.

[0099] On the other hand, in order to minimize the effect of drift, itis necessary to minimize the acquiring time of each of the images forcorrecting the positional displacements and then being added to thelimit. However, the limit is determined by the S/N ratio of the imagesnecessary for detecting the positional displacement. Therefore, if theamount of the drift exceeds a certain value, each of the images itselffor detecting the positional displacement becomes blurred due to drift.When an amount of drift causing such a result is detected, a means fordisplaying that a high resolution image is difficult to be acquired orfor stopping the measurement may be provided. By doing so, it ispossible to solve a problem of uselessly operating the apparatus under astate that acquiring of the sample images is clearly difficult.

[0100] (Embodiment 3)

[0101] A processing flow of FIG. 6 will be described below in detail.

[0102] First Step (S6001):

[0103] A first image F1 for detecting drift is acquired.

[0104] Second Step (S6002):

[0105] An objective image F0 is acquired under an appropriate slowscanning condition. By acquiring the image under such a slow scanningcondition, a high contrast image can be obtained because secondaryelectrons can be generated more compared to the case of fast scanning.

[0106] Third Step (S6003):

[0107] A second image F2 for detecting drift is acquired.

[0108] Fourth Step (S6004):

[0109] A displacement ΔF (ΔFx, ΔFy) between the images F1 and F2 isdetected.

[0110] Fifth Step (S6005):

[0111] Amounts of deformations in the horizontal direction and thevertical direction of the objective image are calculated from the amountof image displacement ΔF.

[0112] Sixth Step (S6006):

[0113] A new image F0′ is formed by deforming the objective image F0.

[0114] Here, the processing of Fifth Step will be described below indetail, referring to FIG. 7.

[0115] Letting the amount of displacement between the drift detectingimages F1 and F2 acquired into the image memory be ΔF (ΔFx, ΔFy), and atime difference between acquiring the image F1 and acquiring the imageF2 be ΔT, drift speeds (Vx, Vy) in X-direction and Y-direction can becalculated by the following equations.

Vx=ΔFx/ΔT, Vy=ΔFy/ΔT

[0116] On the other hand, letting an acquiring time of the objectiveimage F0 be T0, a displacement of view area of the image F0 generatedduring the time period from starting scanning to ending scanning can beexpressed as follows.

[0117] X-direction ΔF0 x=Vx×T0

[0118] Y-direction ΔF0 y=Vy×T0

[0119] Therefore, as shown in FIG. 7, by deforming the objective imageF0 in the image memory by F0 x (Y-direction) and ΔF0 y (X-direction)toward the directions of correcting the drift, it is possible toreproduce the estimated image F0′ which would be obtained if the driftdid not occur.

[0120] In the present embodiment, the images F1 and F2 for detectingdrift are acquired before and after acquiring the objective image F0,respectively, but, of course, the present invention is not limited tothe above. The images F1 and F2 may be successively acquired beforeacquiring the image F0 or after acquiring the image F0. In that case,the deformation is estimated from the displacement between the images F1and F2 at the time when the image F0 is acquired, and then the estimatedimage F0′ can be reproduced.

[0121] Each of the images F1, F2 and F0 is stored in the image memory,and the images F1 and F2 are read out based on judgment on necessity ofimage reproduction, and then the reproduction processing is performed.

[0122] According to the construction described above, the shape of anobserved object deformed by drift can be accurately known.

[0123] Particularly, in the case of slow scanning, the electron beam isbeing irradiated on the sample, and accordingly deformation of thesample image due to charging of the sample becomes large. Therefore,application of the technology of the present embodiment is veryeffective in slow scanning. Further, although the present embodiment hasbeen described on the case of two images for correcting drift and oneimage to be corrected, the present invention is not limited to the aboveand arbitrary number of images may be used.

[0124] Further, in order that the operator judges the necessity of imagereproduction, in the apparatus of the present embodiment, option buttonsfor selecting necessity of image reproduction are provided on agraphical user interface (GUI), as shown in FIG. 16. Although FIG. 16shows the example of performing selection using a pointing device or thelike on the image display unit, the present invention is not limited tothis method. Setting may be performed using another well-known inputsetting means.

[0125] In the observation using an electron beam apparatus such as ascanning electron microscope, there is a need that the sample image ishighly accurately formed. On the other hand, there is also a need thatdamage of the sample is reduced by suppressing irradiation of theelectron beam as low as possible. In the case of the embodiment of theapparatus in accordance with the present invention, the sample image canbe firmed with high accuracy if the deformation of the sample due todrift can be suppressed, but the electron beam scanning for acquiring atleast the images F1 and F2 is further required, which is different fromthe case of simply forming the image F0. That is, since the scanningtime of the electron beam is increased, possibility of the sample damagecaused by7 the electron beam irradiation is increased,

[0126] By providing the option described above, the operator can selectnecessity of the reproduction taking a status of the observed object orthe condition for forming an appropriate sample image intoconsideration, and can form the sample image which the operator desires.

[0127] Further, by making a graph on what extent the deformation iscorrected and registering the graph, the information can be used forsetting the scanning speed and for judgment of necessity of driftcorrection. Further, by storing and displaying number of scanning timesof the electron beam and amount of correction and the amount ofdeformation versus the irradiation time, it is possible to know thesample image is deformed by how long the electron beam is scanned.

[0128] (Embodiment 4)

[0129] An embodiment of applying the drift correction technology toaddition of line profiles will be described below, referring to FIG. 8.

[0130] In general, measurement of dimension of a pattern on a wafer usesa signal distribution (a line profile) which is obtained when anelectron beam is line-scanned on a pattern of a measured object. In acase where the sample is an insulator, high speed scanning is performedin order to prevent disturbance caused by charging. Therefore, since asignal obtained by once of line scanning is bad in the S/N ratio, it isdifficult to perform highly reproducible measurement. Accordingly, ingeneral, signal distributions obtained from several times of scanningare added to form a profile of the measured object. At that time, ifdrift occurs in the direction of line scanning, the added line profilebecomes dull, and accordingly the accuracy of measurement is decreased.

[0131] Therefore, each of the profiles obtained by plural times of linescanning is stored, and positional correction of the profiles isperformed so that the correlation among the profiles becomes highest,and then the profiles are added. In this case, a signal acquired by onceof scanning may be used as each of the profiles before addition.However, when the scanning speed is high, the signal acquired by once ofscanning is too bad in the S/N ratio. Therefore, signals obtained byminimum number of scanning times within a range capable of correlatingamong the profiles are simply added, and the added signal may be used aseach of the profiles before addition. By this method, the problem ofdullness of the profile is improved even if there is drift, and aprofile having a high S/N ratio can be produced. Therefore, it ispossible to perform highly reproducible measurement.

[0132] Further, the apparatus may be constructed in such that thesetting page described in FIG. 16 is also used for selecting thenecessity of positional correction. A semiconductor inspection apparatusor the like may be constructed in such that reliability of themeasurement can be checked later by sounding an alarm or setting a flagto the measurement when an amount of positional correction exceeds athreshold and clearly increases. Therein, in a case where an amount ofcorrection, a added profiler and profiles before adding are stored, orin a case where the line profile is formed based on a two-dimensionalscanned image, the apparatus may be constructed in such that the addedimage or the images before adding are stored and displayed on an imagedisplay unit later.

[0133] According to the construction described above, when a measuredobject is erroneously measured in a repetitive pattern or the like wherepatterns of the same type are adjacently arranged, the erroneousmeasurement can be easily checked.

[0134] Particularly, in the case where a line profile is formed throughone-dimensional scanning, the measurement can be rapidly performedcompared to the case of two-dimensional scanning. However, it isimpossible to check the accuracy of measurement by referring the sampleimage. In the present embodiment, the line profile can be formed withhigh accuracy even in the case of one-dimensional scanning by which themeasurement can be rapidly performed. For example, even in an apparatusmeasuring length of a pattern based on a line profile, the length can bemeasured with high accuracy based on the line profile formed with highaccuracy.

[0135] In a case where a pattern width of a line pattern havingroughness is measured, the measuring length range is expanded towarddirections perpendicular to the direction of measuring length, andmeasurement of length based in the line profile is performed using aplurality of different positions within the measuring length range.Then, the plurality of obtained measured length values are averaged, orthe dispersion values of roughness are measured based on the pluralityof measured length values obtained within the expanded measuring lengthrange. The present embodiment is also applicable to this case.

[0136] For example, by performing addition of line profile with theabove-mentioned positional correction for each of the plurality of thelength measuring positions, and then by measuring the average value orthe dispersion value of roughness, these resultant values can beobtained with high accuracy.

[0137] Even in a case where the line profiles are displaced depending onthe length measuring position due to charge-up or the like, the addedline profile can be appropriately formed and the length can beaccurately measured by performing positional correction, using onereference line profile, to the line profiles in the other positions, notby performing positional correction for each of the plurality of thelength measuring positions. According to the construction describedabove, reliability evaluation of electric property of a semiconductorelement pattern can be easily realized regardless of existence ofroughness.

[0138] Further, in the case of measuring lengths of a plurality ofpositions, when a measured length value of one of the positions isextremely different from the measured length values of the other of thepositions, there is a possibility that a part of the line pattern isextremely thinned, or that a failure of the length measurement occurs.In such a case, the apparatus may be constructed in such that an errormessage is output or that the measured results such as the sample imageand the line profile are registered together with the measuringconditions so as to check the results later by read out the data.

[0139] (Embodiment 5)

[0140]FIG. 9 is a graph for explaining an example of estimating anaccurate dimension from time-varying pattern dimensions by repeatingmeasurement of the same pattern plural times. The object to be measuredusing an electron beam is damaged to be shrunk or evaporated dependingon the material by irradiation of the electron beam. In such a case,since the pattern dimension is decreased as the amount of beamirradiation increases, the measurement itself is an error cause.

[0141] In order to evaluate the correct dimension by estimating theerror caused by the measurement itself, the same pattern is measuredplural times. Since the amount of beam irradiation is increased inproportion to number of measuring times, deformation of the pattern isalso increased as the number of measuring times is increased. Therefore,the pattern dimension before irradiating the beam or before shrinking atstarting the beam irradiation can be estimated by obtaining therelationship between the number of measuring times (in proportion to theamount of beam irradiation) and the dimension measured value. In theembodiment of the apparatus in accordance with the present invention, asequence for automatically executing the above-described dimensionestimation is installed.

[0142] The apparatus may be constructed in such that in order to judgelater whether or not the dimension estimation is correctly performed, atable graphing number of measuring times versus measured value isstored, and then output to the display unit or an external output unit.For example, in a case where an observed object is shrunk and at thesame time drift also occurs, the dimension estimation may be notappropriately performed by influence of the drift, the operator cancheck by referring to the above-described graph whether or not thedimension estimation is correct. By storing a sample image obtained atthat time corresponding to the stored graph, the correctness of thedimension estimation can be checked referring to the sample image.

[0143] The apparatus may be constructed in such that when theabove-described graph records an abnormal trend, the graph isselectively stored or a preset flag is set. For example, when anabnormal change is observed in a graph expressing the trend of dimensionchange, something may occur in the electron beam apparatus at that time,and accordingly the dimension measurement may be not correctlyperformed. If the apparatus is constructed so that the graph or thesample image can be selectively checked at that time, the operator canefficiently check the correctness of the dimension estimation withoutperforming useless check.

[0144] Although the abscissa of the graph expresses “number of measuringtimes” in the present embodiment, the abscissa may express anotherparameter such as “number of scanning times” or “time”. The ordinate isnot limited to express “measured value” either, and the ordinate mayexpress a ratio of a measured value to a normal value (a design value).

[0145] By forming a dummy pattern having a condition equivalent to ameasured object pattern at a position near the measured object patternwhen the present embodiment is applied to an apparatus for measuringlength of a semiconductor pattern, measurement of length can beaccurately performed without shrinking the pattern which affectoperation of the semiconductor element.

[0146] (Embodiment 6)

[0147]FIG. 10 is a view showing an example in which images, each ofwhich has number of pixels larger than number of pixels of an objectiveimage, are acquired, and displacements among the acquired images arecorrected. In the present embodiment shows a case where the number ofpixels of the objective image is, for example, 512×512 pixels. In thisexample, the number of pixels of the acquired image is 1024×1024 pixels.When the acquired images are added by correcting the positionaldisplacements, there appears a region which can not be used as theobjective image due to displacement among the images. In the presentembodiment, the images each having a region wider than the number ofpixels of the objective image are acquired in advance, and a region of512×512 pixels in the central portion is cur out after adding theacquired images to obtain the final objective image.

[0148] Since such slightly larger images are acquired, as describedabove, it does not occur that the peripheral portion of the final imageis lost by being cut off when drift occurs.

[0149] (Embodiment 7)

[0150]FIG. 11 shows an embodiment in which images are added by removingan abnormal image. In a case where an abnormally displaced image or anabnormally blurred image is formed by a sporadic disturbance duringacquiring a plurality of images, or in a case where images acquiredafter acquiring a specific image show abnormal contrast due to chargeduring irradiating the beam, the abnormal image can be removed from theoriginal images to be added by detecting the abnormality through imageprocessing of these images. In regard to displacement, the abnormalitycan be detected by presetting an amount of displacement of view area tobe judged as abnormal. In regard to blur, the abnormal image can beremoved by executing image differential processing or the like, andsetting a threshold to be judged as abnormal. In regard to the abnormalcontrast, the abnormal image can be removed by judging on a histogram orby judging on abnormal decrease in the value of correlation with anotherimage after correcting view area. By removing the abnormal informationas described above, high resolution image can be stably acquired even ifan unexpected cause occurs.

[0151] Although the image judged to be abnormal can be removed, in orderto search the cause of abnormality later, the image judged to beabnormal is stored in the image memory together with the opticalconditions (acceleration voltage of the electron source, emissioncurrent and so on) at the time when the abnormality is recognized, orbefore and after the time when the image is judged to be abnormal.According to the construction described above, it is easy to check whatreason the abnormal image is produced by. For example, if the timingthat over current flows to the cathode of the electron source agreeswith the timing that the abnormal image is produced, the cause exists inthe electron source, which can be used as an index of replacing of theelectron source.

[0152] Changes of current and voltage applied to the optical elementsuch as the extracting electrode, the acceleration electrode or thescanning coil of the electron microscope are displayed by a time chart,and the timing that the abnormality occurs is superposed on the timechart. By doing so, the operator can visually specify the cause.

[0153] The abnormal frame removing technology explained by the presentembodiment can be applied to the line profile addition explained inEmbodiment 4.

[0154] Although the example of mainly automatically removing theabnormal image has been described in the present embodiment, the presentinvention is not limited to the above. For example, it is possible toprovide a function that images before adding are displayed on the imagedisplay unit, and an image judged to be abnormal by the operator can beselectively removed. Therein, if the apparatus is constructed in suchthat some of images can be selected using a pointing device or the likefrom the plurality of images before adding arranged and displayed on theimage display unit, the operator can be visually select images to beremoved from the plurality of images before adding. The apparatus may beconstructed in such that not only the images before adding aredisplayed, but also the plurality of added images are displayed in orderto ascertain abnormal images using the images having a some degree ofS/N ratio.

[0155] (Embodiment 8)

[0156]FIG. 12 shows an embodiment in which view area displacements amonga plurality of images acquired by a plurality of image signals arecorrected, and then the images are added. For example, when an addedimage using reflected electron signal is tried to be acquired, a lot offrames must be acquired for each of the original images because theamount of the reflected electron signal is generally little. The reasonis that if an original image is formed by acquiring a small number ofimage frames acquired by the small signal amount, the view areadisplacement among the images can not detected because the S/N ratio ofthe original image is extremely decreased. On the other hand, if numberof the original image frames is increased, the original image itself isblurred due to drift because the time acquiring the original imagesbecomes long. In the present embodiment, the original images areacquired by the reflected electron signal, and at the same time originalimages having a good S/N ratio are acquired using secondary electronsignal, and view area displacement among the original images acquired bythe secondary electron signal is detected, and then the amount of thedetected view area displacement is applied to the view area displacementamong the plurality of images obtained by the reflected electron signal.

[0157] Since the secondary electron image and the reflected electronimage are acquired at the same time, the view area of the reflectedelectron image completely agrees with the corresponding secondaryelectron image. Therefore, the view area displacement of the originalreflected electron images having a bad S/N ratio can be accuratelycorrected through the method of the present embodiment. Since thesecondary electron signal image having a high S/N ratio is used as theimage for detecting the view area displacement, number of framescomposing the original image can be minimized. Therefore, the originalimage itself is not blurred by drift. As examples of signal having a badS/N ratio, there are, for example, X-ray signal and sample absorptioncurrent. The embodiment of the present invention can be applied tovarious kinds of signals. Particularly, in a case where an elementdistribution (an X-ray image) of a thin film sample is acquired withhigh resolution, the secondary electron signal in the present embodimentmay be replaced by transmission electron signal. In general, occurrenceof X-rays scattering inside a sample can be prevented by making thesample into a foil having a thickness of several tens nm, andaccordingly a high resolution element distribution image can beobtained.

[0158] As the detection system for detecting secondary electrons andreflected electron at the same time, a construction shown in FIG. 17 isconsidered. According to this construction, two kinds of electrons(reflected electrons 1706, secondary electrons 1707) emitted from asample 1705 can be detected at the same time using a reflected electrondetector 1703 and a secondary electron detector 1704 arranged at anupper position and at a lower position of an objective lens 1702 forfocusing a primary electron beam 1701, respectively.

[0159] Further, secondary electrons and reflected electrons can bedetected together using a detection system shown in FIG. 18. In the caseof the construction of FIG. 18, secondary electrons and reflectedelectrons 1803 are accelerated by a retarding voltage 1802 applied to asample 1801, and collide against a secondary electron convertingelectrode 1805 arranged above an objective lens 1804. At the collision,the accelerated secondary electrons and the accelerated reflectedelectrons 1803 produce secondary electrons 1806, and the secondaryelectrons 1806 are attracted to a secondary electron detector 1807 to bedetected.

[0160] An energy filter 1808 is applied with an energy filter voltage1809 which is equal to or slightly higher than the retarding voltage1802 applied to the sample. By applying such a voltage, only thereflected electrons are selectively pass through the energy filter 1808.

[0161] In the construction described above, the secondary electrons andthe reflected electrons are alternatively acquired by switching thevoltage of the energy filter 1809 on-off or strong-weak every acquiringof predetermined number of two-dimensional image frames. Then, thepositional displacement is detected using the secondary electron images,and the positional displacement of the reflected electron image iscorrected using the detected positional displacement information, andthen the reflected electron image is stored in the image memory. Bydoing so, in the scanning electron microscope employing the retardingtechnology, the reflected electron image without blur can be obtained.Although the reflected electrons and the reflected electrons are clearlyseparated in the present embodiment, the present invention is notlimited to the above. The amount of electrons detected by the secondaryelectron detector 1809 may be increased by applying an energy filtervoltage 1809 lower than the retarding voltage 1802 to the energy filter1808. Since most of the electrons emitted from the sample have energysmaller than 50 eV, number of frames composing the original image can beminimized by using electrons having energy smaller than 50 eV for theimages for detecting the view area displacement. The applied voltage tothe energy filter 1808 may be changed depending on the purpose ofanalysis.

[0162] The reflected electron detector and the secondary electrondetector are not limited to those described in the present embodiment,but various types of detectors may be employed. Although the X-raydetector has not been illustrated, all of the existing X-ray detectorsare applicable.

[0163] (Embodiment 9)

[0164]FIG. 13 shows an embodiment in which positional displacements of aplurality of acquired images are corrected only in a specified directionon a sample surface, and then the images are added. In a case where animage has a pattern only in a specified direction in the image,positional displacement in a direction perpendicular to the pattern canbe detected with high accuracy, but accuracy of detecting positionaldisplacement in a direction parallel to the pattern is extremely low. Inregard to such an image, by adjusting view areas only in the directionperpendicular to the pattern and adding the images, the error in theview area adjusting can be reduced. The direction of the pattern can bespecified by analysis of frequency components of the image or lineprofiling of the image by binarization.

[0165] In an apparatus for measuring line width of a pattern on asemiconductor wafer, accuracy of a result of length measurement can bemaintained even when view area displacement is corrected only aspecified direction as described above. Most of patterns on asemiconductor wafer are formed in linier shapes, and line widths ineverywhere on a single line pattern are almost the same. Therefore,measurement of length can be accurately performed unless displacementoccurs only in the direction perpendicular to the pattern.

[0166] In a case where an objective image is a line pattern, and thereis a view area displacement shown in FIG. 20 (a) between two frames ofthe images to be added, the relationship between shifting amount anddegree of agreement becomes as shown in FIG. 20(b). Referring to FIG.20(b), blur in the added image is corrected by overlapping the imagesunder a condition of maximizing the degree of agreement, but thecondition of maximizing the degree of agreement exists not only at oneposition, but at positions distributed in a line shape. Accordingly, thecondition overlapping the images (the condition of maximizing degree ofagreement) can not determine uniquely. Therefore, since the blur of thepattern can be corrected with the minimum shifting amount between theimages when the shifting direction of the image is selected in thedirection perpendicular to the pattern, there is an effect in that theeffective view area of the added image is maximized.

[0167] (Embodiment 10)

[0168]FIG. 14 shows a method of detecting the amount of positionaldisplacement and adding the corrected images. In an input image 1401 andan input image 1402, a region 1403 having an adequate size is put, forexample, in the central portion of the input image 1401, and templatematching is performed to the input image 1402 using the area 1403 as atemplate. Assuming that a region 1404 matches with the region 1403 asthe result, the region 1403 and the region 1404 are overlapped on eachother, and a rectangular region (an AND region) 1405 of overlapping theinput image 1401 and the input image 1402 on each other is set, and aportion not overlapping with the AND region in each of the input image1401 and the input image 1402 is cut off to form apost-position-adjusting input image 1406 or 1407, respectively. Addingprocessing is performed by inputting the post-position-adjusting inputimages 1406 and 1407.

[0169] This example shows the case of two input images, but it is easyto extend to a case of three or more input images. As an example of thetemplate matching, there is a method of executing normalized correlationprocessing between two images based on the following equation, where thesize of the input image is assumed to be 512×512 pixels and the size ofthe template in the center is assumed to be 256×256 pixels. Therein, aposition where the calculated correlation value becomes the maximum isdefined as a matching position.${{r( {x,y} )} = \frac{\lbrack {{N{\sum\limits_{i,j}^{\quad}{P_{ij}M_{ij}}}} - {( {\sum\limits_{i,j}^{\quad}P_{ij}} )( {\sum\limits_{i,j}^{\quad}M_{ij}} )}} \rbrack}{\sqrt{\lbrack {{N{\sum\limits_{i,j}^{\quad}P_{ij}^{2}}} - ( {\sum\limits_{i,j}^{\quad}P_{ij}} )^{2}} \rbrack \lbrack {{N{\sum\limits_{i,j}^{\quad}M_{ij}^{2}}} - ( {\sum\limits_{i,j}^{\quad}M_{ij}} )^{2}} \rbrack}}},$

[0170] Therein, r(x, y) is a correlation value at (x, y), M_(ij) is adensity value at a point (i, j) inside the template, P is a densityvalue at a corresponding point (x+1, y+1) of the input image, and N isnumber of pixels of the template.

[0171]FIG. 15 shows another embodiment of a method of adding correctedimages. Similarly to FIG. 14, in an input image 1401 and an input image1402, a region 1403 having an adequate size is put, for example, in thecentral portion of the input image 1401, and template matching isperformed to the input image 1402 using the area 1403 as a template.Assuming that a region 1404 matches with the region 1403 as the result,the region 1403 and the region 1404 are overlapped on each other, and arectangular region (an OR region) 1501 including both of the input image1401 and the input image 1402 is set, and a portion not overlapping withthe OR region in each of the input image 1401 and the input image 1402is added, and each of the added portions is filled with number of pixelsof 0 or an average value of each of the input images to form apost-position-adjusting input image 1502 or 1503, respectively. Addingprocessing is performed by inputting the post-position-adjusting inputimages 1502 and 1503. This example shows the case of two input images,but it is easy to extend to a case of three or more input images.

[0172] (Embodiment 11)

[0173] Description will be made below on an example in which the driftcorrection technology in accordance with the present invention isapplied to automatic operation of a semiconductor inspection scanningelectron microscope. In general, in order to automatically operate thesemiconductor inspection scanning electron microscope, a recipe file towhich information such as measuring positions and observing conditionsis registered is formed in advance, and then measurement positioning,observation and measurement are performed according to the file. In thepresent method, an environment set before executing the recipe file isregistered. FIG. 19 shows a recipe execution environment page.

[0174] Main sequence of executing the recipe is as follows. That is,initially, alignment for detecting a position of a wafer on a stage isexecuted. At that time, image recognition is performed according to animage registered at forming the recipe. Next, the wafer is moved to themeasuring position using the stage, and an image is acquired with acomparatively low magnification. Positioning of the measured pattern(called as addressing) is performed with high accuracy by imagerecognition, and pattern dimension measurement is performed byelectrically deflecting the electron beam and zooming up to themeasuring magnification. Automatic focus adjustment is performed beforethe positioning of the pattern or before the measurement.

[0175] When test execution of the recipe or in a case where there are aplurality of measured wafers, an amount of drift during the time periodfrom acquiring of an image for positioning the pattern to acquiring animage for measurement is measured for each of the measured points usingthe first wafer. In the case of alignment, an amount of drift at severalminutes after the alignment is measured and stored. At executing therecipe, an amount of drift at each of the measured points or thealignment point is added to the image as an offset after positioning. Inthe case of addressing, the electron beam is deflected to a positionadded with the offset and the magnification is zoomed up to themeasuring magnification. By doing so, the drift after positioning can bereduced, and the plurality of samples can be measured with highthroughput because it is unnecessary to detect the amount of drift atthe actual measurement using the recipe or at measuring the secondwafers and wafers after the second. Whether or not the drift correctionis executed at alignment, at addressing or at measurement is judged byON or OFF of a drift correction switch 1901 for alignment, a driftcorrection switch 1902 for addressing or a drift correction switch 1903for measurement, respectively.

[0176] By providing the environment setting page to be described in thepresent embodiment, it is possible to set a concrete method of driftcorrection which changes depending on a measurement condition and astatus of a sample.

[0177] In recent manufacturing and inspection of semiconductors, aplurality of semiconductor wafers are usually dealt by the cassette unitby containing the semiconductor wafers in a cassette. An apparatus forcontinuously measuring such a plurality of measured objects is providedwith a means for selecting whether or not drift correction is performedbased on an amount of correction registered at forming the recipe and ameans for selecting whether or not drift correction is performed basedon an amount of drift actually measured each wafer. By constructed asdescribed above, when there is individual difference of thesemiconductor wafers in the cassette, the operator judges whether or notthe measurement accuracy takes precedence over the throughput, and theselection can be reflected to the measurement.

[0178] In a case of performing offset correction, a means for selectingwhether or not offset correction is performed based on a valueregistered at forming the recipe and a means for selecting whether ornot offset correction is performed using a value used for detecting theamount of drift in the first wafer in the cassette and registered areprovided. By constructed as described above, when there is amanufacturing error between a test pattern or a design value and anactual pattern, the operator judges whether or not the measurementaccuracy takes precedence over the throughput, and the selection can bereflected to the measurement.

[0179] Although the above description is the example in which theoperator selects the concrete correcting method, the present inventionis not limited to the above. For example, it is possible to provide asequence which automatically sets the concrete method described above byinputting a magnitude of manufacturing error or presence ofmanufacturing error.

[0180] Although the above embodiments have been described on the casesof using the scanning electron microscope, the present invention is notlimited to the scanning electron microscope. The present invention canbe applied to a charged particle beam apparatus of another type in whicha sample image is displaced due to some drift producing cause.

What is claimed is:
 1. A method of forming a sample image by scanning acharged particle beam on a sample and forming an image based onsecondary signals emitted from said sample, the method comprising thesteps of: forming a plurality of composite images by superposing aplurality of images obtained by a plurality of scanning times; andforming a further composite image by correcting positional displacementsamong said plurality of composite images and superposing said pluralityof composite images.
 2. A method of forming a sample image by scanning acharged particle beam on a sample and forming an image based onsecondary signals emitted from said sample, the method comprising thesteps of: two-dimensionally scanning said charged particle beam on saidsample; detecting secondary signals emitted from a scanned region;forming image data based on said detected secondary signals; forming aplurality of composite image data items formed by superposing saidplurality of image data items; detecting positional displacements insaid two-dimensional directions among said plurality of formed compositeimage data items; and forming an image by correcting said detectedpositional displacements.
 3. A charged particle beam apparatuscomprising a charged particle source; a deflector for scanning a chargedparticle beam emitted from said charged particle source; and a detectorfor detecting secondary signals emitted from a scanned region of saidcharged particle beam, a sample image being formed based on saidsecondary signals detected by said detector, which further comprises: acontrol unit which forms a plurality of image data items based on saiddetected secondary signals, and forms a plurality of composite imagedata items by superposing said plurality of image data items, andcorrects displacements among said plurality of composite image dataitems, and then superposes said plurality of composite image data items.4. A charged particle beam apparatus according to claim 3, wherein saidplurality of composite image data items are formed by the unit oftwo-dimensional scanning performed by said deflector.
 5. A chargedparticle beam apparatus according to claim 3, wherein said plurality ofcomposite image data items are acquired in different timing from oneanother.
 6. A charged particle beam apparatus according to claim 3,which further comprises an image shift deflector for shifting a scanningregion of said charged particle beam; and a sample stage for shiftingsaid sample, wherein said control unit detects directions ofdisplacements among said images, and operates said image shift deflectorand/or said sample stage so that said scanning regions may be positionedtoward directions canceling said displacements.
 7. A charged particlebeam apparatus according to claim 6, wherein said control unit shiftssaid scanning region using said sample stage and/or said image shiftdeflector when an amount of said displacement is larger than a presetvalue, and shifts said scanning region using said image shift deflectorwhen an amount of said displacement is smaller than the preset value. 8.A charged particle beam apparatus according to claim 3, which furthercomprises an image sharpening means for sharpening said image to besuperposed, and a displacement between said image data items is detectedbased on image data sharpened by said image sharpening means.
 9. Acharged particle beam apparatus according to claim 3, wherein number ofpixels of said composite image data formed by superposing said pluralityof image data items is larger than number of pixels of the image databefore being superposed.
 10. A method of forming a sample image byscanning a charged particle beam on a sample and forming an image basedon secondary signals emitted from said sample, the method comprising thesteps of: two-dimensionally scanning said charged particle beam on saidsample; detecting secondary signals emitted from a scanned region;forming at least three images based on said detected secondary signals;by using at least two of said three images, detecting deformation orpositional displacement in the other one of said three images; andcorrecting deformations or displacements of said image based on saiddetected deformation.
 11. A charged particle beam apparatus comprising acharged particle source; a deflector for scanning a charged particlebeam emitted from said charged particle source; and a detector fordetecting secondary signals emitted from a scanned region of saidcharged particle beam, a sample image being formed based on saidsecondary signals detected by said detector, which further comprises: acontrol unit for two-dimensionally scanning said charged particle beamon said sample; detecting secondary signals emitted from a scannedregion; forming at least three images based on said detected secondarysignals; by using at least two of said three images, detectingdeformation or positional displacement of the other one sheet; andcorrecting deformations or displacements of said image based on saiddetected deformation.
 12. A method of measuring a dimension of sample byscanning a charged particle beam on a sample, and forming a line profilebased on secondary signals emitted from said sample, and measuring adimension of a measured object on said sample from said line profile,the method comprising the steps of: scanning said charged particle beamon said sample; detecting secondary signals emitted from a scannedpositions; forming a plurality of line profiles based on said detectedsecondary signals; detecting positional displacements among saidplurality of line profiles; superposing said plurality of line profileby correcting said displacements, and measuring a dimension of saidmeasured object based on said composite line profile.
 13. A chargedparticle beam apparatus comprising a charged particle source; adeflector for scanning a charged particle beam emitted from said chargedparticle source; and a detector for detecting secondary signals emittedfrom a scanned region of said charged particle beam, a line profilebeing formed based on said secondary signals detected by said detector,a dimension of a measured object being measured based on said lineprofile, which further comprises: a control unit for scanning saidcharged particle beam on said sample; detecting secondary signalsemitted from a scanned positions; forming a plurality of line profilesbased on said detected secondary signals; detecting positionaldisplacements among said plurality of line profiles; superposing saidplurality of line profile by correcting said displacements, andmeasuring a dimension of said measured object based on said compositeline profile.
 14. A method of measuring a dimension of sample byirradiating a charged particle beam on a sample, and measuring adimension of a measured object formed on said sample based on secondarysignals emitted from said sample, the method comprising the steps of:measuring decreasing change of a dimension of said measured object whichdecreases as said sample is being irradiated with said charged particlebeam; and calculating a dimension of said measured object before or atstarting irradiating said charged particle beam from said decreasingchange.
 15. An electron beam apparatus comprising a charged particlesource; a deflector for scanning a charged particle beam emitted fromsaid charged particle source; and a detector for detecting secondaryelectrons emitted from a scanned region of said electron beam, whichfurther comprises: a reflected electron detector for detecting reflectedelectrons emitted from said sample and/or an X-ray detector fordetecting X-rays emitted from said sample; and an image display unit fordisplay a sample image based on reflected electrons detected by saidreflected electron detector or X-rays detected by said X-ray detector,wherein a shifting amount of a secondary electron image detected by saidsecondary electron detector is detected, and a position of a reflectedelectron image or an X-ray image displayed on said image display unit iscorrected based on said shifting amount of the secondary electron image.16. A method of forming a sample image by scanning a charged particlebeam on a sample and forming an image based on charged particles emittedfrom said sample, the method comprising the steps of: scanning saidcharged particle beam on said sample plural times; obtaining a pluralityof images based on detection of secondary signals emitted from saidsample based on said plural times of scanning; removing images havingabnormality from said plurality of images; and superposing saidplurality of images excluding said images having abnormality to form acomposite image.
 17. A charged particle beam apparatus comprising acharged particle source; a deflector for scanning a charged particlebeam emitted from said charged particle source; and a detector fordetecting secondary signals emitted from a scanned region of saidcharged particle beam, a sample image being formed based on saidsecondary signals detected by said detector, which further comprises: acontrol unit which forms a plurality of image data items based on saiddetected secondary signals, and removing image data items havingabnormality from said plurality of image data items, and superposingsaid plurality of images excluding said image data items havingabnormality.
 18. A method of forming a sample image by scanning acharged particle beam on a sample and forming an image based on chargedparticles emitted from said sample, the method comprising the steps of:selectively correcting displacements in a specific direction among aplurality of images obtained by plural times of scanning; andsuperposing said plurality of corrected images to form a compositeimage.
 19. A method of forming a sample image according to claim 18,wherein said specific direction is a direction intersecting at rightangle with a longitudinal direction of a line pattern formed on saidsample.
 20. A charged particle beam apparatus comprising a chargedparticle source; a deflector for scanning a charged particle beamemitted from said charged particle source on a sample; a detector fordetecting secondary signals emitted from a scanned region of saidcharged particle beam, an image memory for storing a sample image formedbased on charged particles detected by said detector, wherein said imagememory is constructed so as to add a plurality of sample images and thento store said added sample image, and comprises means for correctingdisplacements of said sample images to be added and performing saidaddition; and means for setting a condition of said addition.
 21. Acharged particle beam apparatus comprising a charged particle source; adeflector for scanning a charged particle beam emitted from said chargedparticle source; and a detector for detecting secondary signals emittedfrom a scanned region of said charged particle beam, a sample imagebeing formed based on said secondary signals detected by said detector,which further comprises: means for forming N0 frames of image data itemsbased on said detected secondary signals, and for forming N1 frames ofcomposite image data items by superposing said N0 frames of image dataitems; and setting means for setting said N0 and said N1.